Yarn feeders are known, of the so-called “positive” type, in which the yarn originating from a distaff is repeatedly wound between a motorized spool and a spacer pin with an axis that is slightly oblique with respect to the axis of the spool. By making the spool rotate in the direction of unwinding, the yarn is fed to a textile machine downstream, e.g., a knitting machine.
The function of the spacer pin is to maintain the yarn loops wound on the spool, axially spaced apart from each other.
The yarn tension is continuously monitored by a load cell connected to a control unit. The latter, on the basis of the signal received from the load cell, modulates the speed of the spool so as to keep substantially constant at a desired level the tension of the yarn fed to the machine downstream, to the advantage of the quality of the knitting produced.
As is known, in knitting processes, often the yarn feeder has to recover a part of the yarn that was previously supplied to the machine downstream.
In these cases, a gathering device can be provided upstream of the feeder. During recovery, the yarn winding spool is made rotate in the opposite direction with respect to the direction for feeding and, simultaneously, the gathering device is activated in order to keep the yarn upstream of the spool under tension.
A solution of this type is illustrated, e.g., in EP 1501970 B1, where the gathering device is based on a Venturi tube.
The introduction of a gathering device upstream of the spool implies a not insignificant increase in cost, not least because it needs to be controlled so that it is activated synchronously with the spool.